Switching power converters are widely used in various electronic devices. A switching power converter typically comprises at least one switch and is configured to convert an input voltage to a desired output voltage through controlling the on and off switching of the at least one switch.
One of the most frequently used control methods of switching power converters includes peak current control pulse width modulation method. In brief, a switching power converter which operates with the peak current control pulse width modulation method regulates a main switch of the switching power converter to switch on and off with a substantially constant frequency determined by a system clock signal, and regulates the on time of the main switch in each switching cycle based on a feedback signal indicative of the output voltage and a current sense signal indicative of a switching current flowing through the main switch. A fraction of the on time of the main switch during the total time of an on and off switching cycle may be referred to as an on duty cycle of the switching power converter.
The switching power converter may also needs to limit its output current or the switching current to keep the output current lower than an output current limit threshold (or the switching current lower than a switching current threshold) so that the switching power converter and loads supplied by the switching converter can operate in a safe operation current range. Generally, the limitation of the output current and/or the switching current can be realized through controlling a peak value of the switching current to be lower than a predetermined peak current threshold. However, since there exists a parasitic capacitor between the main switch and a reference ground of the switching power converter, it is necessary to blank a leading edge of the current sense signal indicative of the switching current so that the current sense signal is disabled during a predetermined leading edge blanking (LEB) time, in order to prevent a current limit function from being mis-triggered due to spikes occurred at each leading edge of the current sense signal. Therefore, a minimum duty cycle of the switching converter is required.
For example, for a buck switching power converter, given a period of the system clock signal is 16 μs (i.e. the on and off switching cycle of the main switch is 16 μs) and a 250 ns LEB time is required, it can be deduced that the minimum duty cycle of the buck switching power converter should be 250/1600≈1.5%. Assuming a maximum allowable input voltage of the buck switching power converter is 380V and a 12V output voltage is desired, the on duty cycle should be 12/380, approximately 3%, which is larger than the required 1.5% minimum duty cycle. Thus, the buck switching converter can operate normally in this circumstance. However, at a startup of the buck switching power converter, the instantaneous output voltage is quite small compared with the desired value of the output voltage, therefore the duty cycle theoretically should be smaller than the 1.5% minimum duty cycle. However, the buck switching converter practically operates with the 1.5% minimum duty cycle, resulting in over charge of an inductor of the buck switching converter in each switching cycle, which leads to the inductor current continuously increasing. As a result, the inductor current may exceed a maximum allowable safe operating current in a very short time, causing the buck switching converter to fail to operate normally. In another aspect, the main switch of the buck switching converter (e.g. a metal oxide semiconductor transistor) generally has limited current conduction capability. The continuous increase in the inductor current also means a continuous increase of a switching current that should be flowing through the main switch. This can lead to quick saturation of the main switch and abrupt increase in a drain to source voltage of the main switch, resulting in a higher conduction loss. In this circumstance, the main switch not only has to suffer from high voltage and temperature pressure, but also has the risk of being damaged, which harms the stability and the robustness of the buck switching converter.
Typically used soft-start techniques, such as soft-starting a peak current threshold of the switching current or soft-starting the reference voltage indicative of the desired value of the output voltage can not resolve the above mentioned problems.